Liquid cooled nuclear reactor

ABSTRACT

There is disclosed a nuclear reactor that has a reactor tank enclosing a reactor core and a component tank accommodating a coolant pump and a heat exchanger disposed in a shell which is secured gastight to the top wall of the component tank. The coolant is carried by conduits in both directions between the heat exchanger and the reactor core. The space above the coolant level in the component tank externally of the shell communicates by means of a further conduit with the space above the coolant level in the reactor tank. The space above the coolant level in the reactor tank communicates with the space above the coolant level in the shell by means of a pressure equalizing conduit.

United States Patent [1 1 Miiller Feb. 19 1974 [54] LIQUID COOLEDNUCLEAR REACTOR 3,425,907 2/1969 Bonsel et al. 176/65 3,410,752 11/1968Dell 176/65 X [75] lm'entor- Muller Neuthard 3,395,076 7/1968 Ruppen,Jr. 176/65 Germany [73] Assignee: Gesellschaft fur Kernforschung PrimaryExaminer-Benjamin R. Padgett m.b.H, Karlsruhe, Germany AssistantExaminer-Roger S. Gaither [22] Filed: Mar. 22, 1971 Attorney, Agent, orFirmSpencer & Kaye [21] Appl. No.: 126,855 ABSTRACT [30] ForeignApplication Priority Data There is disclosed a nuclear reactor that hasa reactor tank enclosing a reactor core and a component tank Mar. 21,1970 Germany 2013586 accommodating a coolant pump and a heat exchangerdisposed in a shell which is secured gastight to the top [52] US. Cl176/65, 176/62, 1l7766//6837, wall of the component tank The coolant iscarried by conduits in both directions between the heat ex- Ilftchangerand the reactor core The space above the Fleld of Search 63, 65, 87coolant level in the component tank externally of the shell communicatesby means of a further conduit [56] Reerences C'ted with the space abovethe coolant level in the reactor UNITED STATES PATENTS tank. The spaceabove the coolant level in the reactor 3,151,034 9/1964 Douglass, Jr; eta1. 176/65 X tank communicates with the space above the coolant3,242,981 3/1966 Hutchinson et a1 176/65 X level in the shell by meansof a pressure equalizing 3,182,002 5/1965 Laithwaite et al.... 176/65 dit 3,312,596 4/1967 Grain 176/65 X 3,448,797 6/1969 Chevallier et a1176/65 X 10 Claims, 2 Drawing Figures 1 LIQUID COOLED NUCLEAR REACTORThe invention relates to a liquid cooled nuclear reactor consisting of astationary reactor tank containing the core and a stationary componenttank containing the heat exchangers and the coolant circulation pump,the component tank being connected with the reactor tank by a coaxialtube in which the coolant flows from the core to the heat exchangers andback.

There are two different systems of liquid cooled, especially liquidsodium cooled nuclear reactors:

the pool type combining all important reactor components in one singlereactor tank and the loop type in which all the major and importantreactor components are installed in a tank separate from the reactor andare connected with the reactor by means of a tube system.

Below, the advantages and disadvantages of both sys tems will beinvestigated.

1. Loss of Coolant In the pool concept, the pressurized section of theprimary circuit is arranged completely within the low pressure coolant;hence, the outer tank wall must withstand only the low pressures of thecoolant and the cover gas. For this reason, minor leaks in thepressurized section can be tolerated and for the same reason a loss ofcoolant in the system due to pumps continuing to run is impossible.Besides, leakages from the pool can be prevented by arranging a tightlyfitting containment around the tank in such a way as to prevent thecoolant level in the tank from decreasing to a dangerous level. Inaddition, the probability of leaks is very low in these low pressurevessels which are free from any attachments and tube penetrations.

By contrast, the loop concept offers comparable positive characteristicsonly if a double wall is provided for, at least in the pressurizedsection. However, this concept is not applied because it results inmajor difficulties in design and construction. Hence, if there is a leakin the pressurized section of the circuit, the pumps must immediatelystop or the quick-acting valves must be actuated at once together with areactor scram to prevent excessive loss of coolant.

2. Decayheat Removal In this respect, both systems are almostequivalent, provided that excessive loss of coolant can be prevented.However, the change'from full power operation to' emergency cooling,especially in the case of a major leakage, is easier than in the pooltype plant because the main pumps remain in operation and the large sdium capacity reduces the effect of thermal shocks. In addition, asmaller capacity of the emergency cooling system is sufficient becauseexcess heat can be retained in the sodium pool for some time in theinitial period following a reactor scram.

3. Limitations Imposed on Adjacent Components by the Concept In the loopconcept, the installations above the reactor, especially the refuelingsystems, are practically unimpaired because the primary circuitcomponents are set up a sufficient distance away from the reactor core.By contrast, these parts are influenced in the pool system, especially,if a hot cell is required for refueling. 4. Technical Risk i In thisrespect, the loop typesystem is undoubtedly superior to the 'pool typesystem because it entails much less new problems than the latter does.The problems discussed specifically in connection with the pool typeconcept are these: Reactor tank to be fabricated on site; sealing ofshielded tanks of large diameters; thermal insulation; neutron shieldpositioned in sodium; movable seals; instrumentation, etc. In addition,the loop type concept allows easier assembly work because prefabricatedinternals can be installed under relatively clean conditions.

5. Maintenance Characteristics With respect to maintenancecharacteristics, the loop system is again superior to the pool systembecause all the components are more easily accessible. In addition, thecoolant activity during operation can decay in a separate loop at areduced power level and, moreover, it is possible to shut down singleloops whenever repair work must be done. This is impossible in poolsystem. Moreover, the pool type reactor contains a number of individualcomponents, e.g., movable seals, which are badly accessible.

As is evident from these comparisons, there are a number of reasons infavor of the loop type concept. However, pool type reactors offerdecisive advantages with respect to the problem of leak tightnessbecause they require no quick acting safety installations. Therefore,these advantages should be retained in other designs wherever possible.

Only one nuclear reactor is known (USAEC report ANL-7520, part II) inwhich the intermediate heat echangers and the coolant pumps are combinedin one unit. However, in that design, the outer tank wall is subjectedto the full coolant pressure. Now, the invention has the purpose ofsolving the problem of designing a nuclear reactor on the basis of thisstate of the art, which, as much as possible, combines the advantages ofpool type reactors (e.g., leak tightness and insensitivity to losses ofcoolant) with those of loop type reactors (e.g., good maintenancecharacteristics, and low technical risk) and yet allows a very compact,economical design that can be implemented with simple constructionexpenditure. Moreover, the reactor is to satisfy the most stringentcriteria applied to its components and its operation with respect tosafety in cases of disturbance.

In the invention, this problem is solved by subdividing the amount ofcoolant between the reactor and the component tanks in such a way, atleast during reactor operation, that the component tank is filled withcoolant only in the bottom part and the coolant level in this tankoutside the heat exchangers is kept below the coolant level of thereactor tank. In its upper region, the inner space of the component tankis subdivided into two separate spaces by means of a gastight shellclosing the heat exchanger and extending up to the top; the space abovethe coolant level outside the heat exchanger area is connected with thespace above the coolant level in the reactor tank by a tube equippedwith a pressurizing system. Moreover, the space above the coolant levelin the area of the heat exchanger is connected with the space above thecoolant level in the reactor tank by means of a pressure equalizationline andwith the housing of the coolant pump above and below the coolantlevel by equalization tubes. In a special embodiment of the invention,the coaxial tube connecting the tanks opens into the component tankabove the coolant level; at the point of discharge a corrugated tubecompensator is installed between the outer tube of the coaxial line andthe outer wall of the component tank. The outer tube of the coaxial linein the component tank runs upward above the coolant level into atransverse duct which, in turn, opens into the shell of the intermediateheat exchanger in a gas tight connection at the height of the coolantlevel.

Further details of the invention are explained below on the basis ofFIGS. 1 and 2.

FIG. 1 is a cross section of a reactor with the component tank attachedto it;

FIG. 2 is a top view of a nuclear reactor plant in which three componenttanks are attached to one reactor tank.

The basis principle of the invention is shown in FIG. 1. The reactorcore 1 is arranged in the bottom part of the reactor tank RP. A coaxialtube 4,6 connects one or several stationary component tanks CP with thereactor tank RP. Each component tank CP accomodates the components ofthe primary cooling system, especially the intermediate heat exchangersII-IX and the coolant circuit pumps. The intermediate heat exchanger IHXand the coolant circuit pumps 2 are stationary components attached tothe upper shielding top 19 of the component tank. The outer shell 21 ofthe intermediate heat exchanger IHX is connected gastight with the top19. The coolant pump 2 carries a check valve 3 on the suction side. Thepump takes the coolant in from the component tank CP and forces itthrough an intermediate tube 27 into the inner tube 4 of the coaxialline to the reactor core 1. For thermal equalization, the tube 27 isprovided with articulated compensators 5. Tube 4 forms the centralchannel of the coaxial line of connection. It is equipped with a thermalinsulation not shown in greater detail. The coolant heated in thereactor tank RP flows back under gravity into the component tank CP viathe annular channel 6 of the coaxial line. Passing the transversechannel 7 and the overflow edge 8 it enters into the outer shell 21 ofthe intermediate heat exchanger IHX. The special designs of thetransverse channel 7 and the overflow edge 8 serve the purposes ofsteadying the influx of the coolant into the heat exchanger IHX andpreventing gas bubbles from the cover gas to be carried over. Then thecoolant flows through the intermediate heat exchanger IHX in a downwardflow and collects in the bottom part 22 of the component tank CP.

The thermal expansions occurring in the pressure tube 6,4 between thepump 2 and the inlet plenum of the reactor core 1 are balanced by thearticulated compensators 5 mentioned above. The outer shell 23 of thecoaxial line is routed merely in the wall of the component tank CP. Therequired seal is achieved through axial corrugated tube compensators 9.These corrugated tube compensators are installed so as to be easilyremovable and to prevent a potential leakage from causing a major lossof sodium.

This design allows an unrestricted thermal expansion of the componenttanks CP and the connecting lines 6,4. In order to be able to accomodatealso thermal expansions within the component tank, sliding sleeves 10are installed at several points of the low pressure system, e.g., at thepoint of entry of the pressure line 4 into the coaxial duct 6, in theouter wall of this coaxial duct 6 and in the transverse duct 7 leadingto the intermediate heat exchanger IHX. However, complete sealing is notnecessary at these points.

As is shown in FIG. 1, the axial corrugated tube compensators 9 at thepoint of penetration of the coaxial line 6 through the wall of thecomponent tank C? are located within the inert gas space, whereas theliquid coolant is maintained at a lower level. As a consequence, thermalstresses are largely avoided in these corrugated tube compensators 9 andtheir attachment parts.

The coolant level within the shell 21 of the intermediate heat exchangerII-IX,V IHX, is nearly corresponding to that of the reactor tank, V RP.Only the slight pressure drop A h, between the reactor tank RP and theintermediate heat exchanger IHX causes a small difference. Thesecharacteristics of the coolant level are in line with the slightlyhigher cover gas pressure A h in the component pool CP. For this reason,the cover gas system contains a pressure step-up system 11 besidesequalization tubes 12 connecting the component tank C? with the shell ofthe intermediate heat exchanger IHX. However, the higher cover gaspressure in the component tank CP can be maintained also by means ofspecial pressure control systems which may be parts of the cover gasstorage or the purification systems.

These coolant level pressure conditions are determined by the followingequations:

V II-IX-VCP=Ah +Ah where A h is the pressure drop in the intermediateheat exchanger IHX.

This system is self-regulating. If the coolant flow rate is increased bythe pump 2, VCP will decrease and V RP will increase. As a consequence,in case A h remains constant, the coolant flow rate from the reactortank RP to the component tank CP is increased until the rise in pressureA h, A h; corresponds to the conditions mentioned above. In case of afailure of the pump 2, V CP and VRP will increase and V IHX willdecrease until the difference in levels is equal to A h;,. At the sametime, the flow rate and A h, A h will drop to zero. However, in order tolimit the coolant level fluctuations to an acceptable value in case of Ah remaining constant, a comparatively small value is selected for A hand A h The intermediate heat exchanger shell 21 and the pump housing 24are connected by the coolant and the overflow tube 28. Therefore, thecoolant level of the pump 2 corresponds to the level in the intermediateheat exchanger IHX. Moreover, the packing seal 13 of the pump shaft canoperate under low pressure conditions.

The reactor tank RP and the component tank CP are arranged inindividually shielded vaults 14. A containment 15 closely surroundingthe vaults encloses the tank CP and the coaxial lines 6. The containmentis thermally insulated and is cooled by inert gas on the outer surface.The containment 15 is supported by special suspension systems 25 in sucha way that a differential thermal expansion is balanced while thehorizontal tube section acts as a tensible and compression bar.

FIG. 2 shows a major reactor facility in which three component tanks areattached to one reactor tank. Each component tank CP contains twointermediate heat exchangers IHX and a coolant pump 2. One componenttank, which is shown in an open representation, shows details of theinstallation of the axial corrugated tube compensators and thearrangement of the coolant lines, especially of the two transverse ducts7. Besides the penetrations for the two intermediate heat exchangers IHXand the pump 2 in the shielded top, the component tank G? contains aflap 17 which is closed by a shielded insert and allows easy access tothe inner components. A hot cell 18 for refueling and all maintenanceoperations in the reactor tank can easily be installed in theinterspaces between the tops of the component tanks.

Failure of one coolant pump causes a change in the coolant level and isaccompanied by a decrease of the flow rate in the disturbed primarysub-cooling system, as described above. The check valve in the pumpinlet is closed by the coolant pressure generated by the running pumpsof the other sub-cooling systems. Hence, it is possible to continueoperation of the plant at a reduced power level.

In the case of rupture of a secondary circuit, the coolant temperaturein the component tank containing the failed cooling system will rise.However, the resultant thermal shock will be relatively small as aconsequence of the rather large coolant volume.

In the case of a loss of cover gas pressure A h V CP will rise andV RPwill fall until the differential pressure is equal to A h, A 11 Underthese conditions the axial corrugated tube compensators will contactliquid sodium.

Minor leaks in the pressure tubes can be tolerated. In the case of majorleaks, e.g., the rupture of a whole tube, the reactor must be scrammed.

If there is a leak in the outer wall of the tank within the coolantarea, the coolant will fill the annular gap between the wall of thecontainer and the containment up to a level which is determined by thedifferential pressure between the cover gas in the reactor tank and thecover gas in the shielding vault. The resultant decrease in the coolantlevel can be tolerated.

In the case of a leak in the cover gas area of the component tankthequantity of gas leaking out can be limited by the followingoperations in the disturbed component tank.

The pump is turned off.

The cover gas pressure in the intermediate heat exchanger is increasedto such a level that the coolant level in the transverse duct betweenthe outer space of the coaxial tubes and the shell of the intermediateheat exchanger decreases below the overflow edge 8.

The pressure in the cover gas space of the component tank is reduced andbalanced by an increase in cover gas pressure in the shielded vault.

In each of the disturbances outlined above the removal of the decay heatin the remaining sub-systems at a low pumping speed is guaranteed.However, it is possible also to install separate auxiliary coolingsystems 16 in the reactor tank RC, as shown in dashed lines in FIG. 1.Such an auxiliary system 16 can operate with natural convection on theprimary and secondary sides and will safeguard the necessary amount ofcooling also if there is a complete failure of the pumps in all thecircuits. During normal operation, a check valve 26 in the primary sideof the auxiliary cooling circuit is no danger of inert gas being carriedover in the heat exchanger. The feeding pressure of the circulation pumpis increased, i.e., the suction conditions are improved so that a morecompact pump with a higher speed can be utilized.

Moreover, it is possible to operate the packing seat of the pump shaftpenetration under low pressure.

These advantages and others characterize the invention as an optimaldesign for liquid metal cooled reactors. Especially the leak tightness,which is comparable to a pool type reactor, and the compact design, thediminution of the consequences of a thermal shock, the possibility ofclosing down individual primary subcooling systems, the good maintenanceand repair characteristics of components in the primary cooling system,the simple preheater system and, finally, the free space available forrefueling above the reactor prove to be highly advantageous.

I claim:

1. A liquid cooled nuclear reactor comprising in combination:

a. a stationary reactor tank;

b. a reactor core disposed in said reactor tank;

c. a stationary component tank situated externally of said reactor tankand having an inner space including an upper zone and an upper boundary;

d. a shell disposed in said component tank and connected gastight tosaid upper boundary, said shell subdividing the inner space of saidcomponent tank into two regions;

e. a heat exchanger disposed in said shell;

f. first conduit means interconnecting said tanks for carrying coolantfrom said reactor core to said heat exchanger;

g. second conduit means interconnecting said tanks for carrying coolantfrom said heat exchanger to said reactor core;

h. a coolant circulation pump disposed in said component tank formaintaining a coolant flow between said tanks, whereby at least duringreactor operation, the coolant level in said component tank externallyof said shell being maintained below the coolant level in said reactortank,

i. third conduit means maintaining communication between the space abovethe coolant level in said reactor tank and the space above the coolantlevel externally of said shell in said component tank,

j. a pressurization system communicating with said third conduit means;and

k. a pressure equalizing fourth conduit means maintaining communicationbetween the space above the coolant level in said reactor tank and thespace above the coolant level of said heat exchanger in said shell.

2. Nuclear reactor as claimed in claim 1 in which said coolant pump hasa housing; the space within said shell is connected with the inner spaceof said housing above and below the coolant level in said shell by meansof equalization tubes.

3. Nuclear reactor as claimed in claim 1 in which said first and secondconduit means constitute a coaxial tube line having an outer tube and aninner tube, said outer tube merging into the component tank above thecoolant level therein, said combination further comprising a corrugatedtube compensator installed between the outer tube of the coaxial lineand 'a bounding wall of the component tank.

4. Nuclear reactor as claimed in claim 3 in which the outer tube of thecoaxial line in the component tank above the coolant level leadsvertically upward into a transverse duct merging gastight into the innerspace of said shell at the height of the coolant level therein.

5. Nuclear reactor as claimed in claim 3 in which the circulation pumpdraws coolant from a bottom zone of the component tank, said combinationfurther comprising an intermediate tube connecting said pump with theinner tube of the coaxial line, said intermediate tube extending upwardand is subdivided into several intermediate pieces within the componenttank, said intermediate pieces are interconnected by articulatedcompensators.

6. Nuclear reactor as claimed in claim 3, wherein said outer tubeextends from said reactor tank below the component tanks.

1. A liquid cooled nuclear reactor comprising in combination: a. astationary reactor tank; b. a reactor core disposed in said reactortank; c. a Stationary component tank situated externally of said reactortank and having an inner space including an upper zone and an upperboundary; d. a shell disposed in said component tank and connectedgastight to said upper boundary, said shell subdividing the inner spaceof said component tank into two regions; e. a heat exchanger disposed insaid shell; f. first conduit means interconnecting said tanks forcarrying coolant from said reactor core to said heat exchanger; g.second conduit means interconnecting said tanks for carrying coolantfrom said heat exchanger to said reactor core; h. a coolant circulationpump disposed in said component tank for maintaining a coolant flowbetween said tanks, whereby at least during reactor operation, thecoolant level in said component tank externally of said shell beingmaintained below the coolant level in said reactor tank, i. thirdconduit means maintaining communication between the space above thecoolant level in said reactor tank and the space above the coolant levelexternally of said shell in said component tank, j. a pressurizationsystem communicating with said third conduit means; and k. a pressureequalizing fourth conduit means maintaining communication between thespace above the coolant level in said reactor tank and the space abovethe coolant level of said heat exchanger in said shell.
 2. Nuclearreactor as claimed in claim 1 in which said coolant pump has a housing;the space within said shell is connected with the inner space of saidhousing above and below the coolant level in said shell by means ofequalization tubes.
 3. Nuclear reactor as claimed in claim 1 in whichsaid first and second conduit means constitute a coaxial tube linehaving an outer tube and an inner tube, said outer tube merging into thecomponent tank above the coolant level therein, said combination furthercomprising a corrugated tube compensator installed between the outertube of the coaxial line and a bounding wall of the component tank. 4.Nuclear reactor as claimed in claim 3 in which the outer tube of thecoaxial line in the component tank above the coolant level leadsvertically upward into a transverse duct merging gastight into the innerspace of said shell at the height of the coolant level therein. 5.Nuclear reactor as claimed in claim 3 in which the circulation pumpdraws coolant from a bottom zone of the component tank, said combinationfurther comprising an intermediate tube connecting said pump with theinner tube of the coaxial line, said intermediate tube extending upwardand is subdivided into several intermediate pieces within the componenttank, said intermediate pieces are interconnected by articulatedcompensators.
 6. Nuclear reactor as claimed in claim 3, wherein saidouter tube extends from said reactor tank below the coolant leveltherein.
 7. Nuclear reactor as claimed in claim 1 in which an additionalemergency cooling system is installed in the reactor tank.
 8. Nuclearreactor as claimed in claim 1 in which several component tanks areconnected with one reactor tank.
 9. Nuclear reactor as claimed in claim1 in which several heat exchangers and coolant pumps are arranged ineach component tank.
 10. Nuclear reactor as claimed in claim 1, saidcombination further comprising a hot cell for refueling arranged abovethe reactor tank between the tops of the component tanks.